Digital liquid level sensing apparatus

ABSTRACT

A digital liquid level sensing apparatus for detecting variations in the dielectric of a substance being sensed. The apparatus includes a capacitive element array including a plurality of individual (i.e., segmented) input plates positioned along an axis of measurement of a fluid to be detected. The array also includes a common output plate having a length sufficient to span the entire accumulated length of the input plates. A controller sequentially applies DC excitation pulses to the input plates which cause a series of output currents to be coupled onto the output plate. The output currents are input to a current-to-voltage amplifier which generates a series of corresponding analog output voltages. The analog output voltages are then input to a peak voltage detector circuit to generate a series of peak voltage signals representative of the magnitudes of the analog output voltages. The controller converts each of the peak voltage signals into a corresponding digital value and stores each of the digital values in an on-board memory. The controller then sequentially compares each of the values against at least one predetermined reference value indicative of an output produced by an input plate disposed in air until a predetermined difference is detected between the reference value and any one of the stored digital values. This indicates a predetermined difference in the dielectric, thus indicating that a corresponding input plate is at least partially submerged in fluid.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to liquid level sensors, and more particularly toa digital liquid level sensing apparatus incorporating a dielectricconstant differentiator for detecting variations in the dielectric ofsegmented portions of a capacitive probe.

2. Discussion

Liquid level sensors are used in a variety of applications to sensefluid levels in reservoirs where it is important or desirable toperiodically or continuously monitor the level of the fluid within areservoir. One form of liquid level sensor employs a capacitive probehaving a pair of continuous, elongated elements (i.e., plates)positioned on a substrate of the probe. This form of sensing systemmakes use of the difference in the dielectric of air from variousliquids. In such systems, some means is provided for generating a signalwhich is applied to one plate of the probe. The overall capacitance ofthe capacitor formed by the two plates, and thus the magnitude of thesignal coupled onto the other one of the plates on the probe, willchange as the percentage of the probe submerged in a fluid, and thus thetwo plates thereof, changes. Thus, the magnitude of the signal coupledonto the output plate of the probe can provide a relative indication ofthe area of the probe which is submerged in fluid and/or exposed in air.

Many prior developed systems incorporating capacitive probe technologyhave involved going to great lengths to fully characterize thedielectric constant of the substance whose level is being monitored inan effort to effect an accurate measurement of the level of thesubstance within a given reservoir. In some instances, such approacheshave involved making some form of in-situ measurement of the dielectricconstant. Other approaches attempt to avoid the affects of the varyingdielectric constant by attempting to remove the variation from themeasurement. This is highly desirable because the dielectric constant ofa given substance may vary to a significant degree when the substanceexperiences severe temperature changes or contamination from othersubstances which enter the reservoir. Thus, the overall accuracy of manysuch liquid level sensing systems incorporating capacitive probetechnology can be greatly adversely affected by changes in thedielectric constant of the substance being measured as the compositionof the subject is subjected to various environmental factors (e.g.,temperature) and as the composition of the substance varies over aperiod of time.

One application where liquid level sensors are particularly desirable iswith automotive vehicles. Recently there has been increasing interest inmonitoring an even greater number of different fluids associated withmotor vehicles to ensure that such fluids remain at optimum levels. Forexample, there has been increasing interest in incorporating sensingapparatus for sensing engine coolant levels, transmission fluid levelsand differential case fluid, just to name a few. The use of liquid levelsensing apparatus with such fluids, however, presents a number ofproblems due to the extreme environmental changes which such a sensingapparatus must be able to tolerate, as well as the cost constraintswhich must be met in order for the apparatus to be economically massproduced without adding significantly to the overall price of thevehicle.

Recently released requirements, typical of the auto industry at large,for a fuel level sensor are listed below to provide an idea of thestringency of present day operational parameters which a fuel levelsensor suitable for use in automotive applications must meet:

    ______________________________________                                        A.    Temperature Range - (-)40° C. to 150° C.                  B.    Life - 20 Years                                                         C.    Response Time - preferably in the area of about or                            reasonably close to 15 milliseconds                                     D.    Accuracy - 0.5 gallons minimum                                          0.1 gallons preferred                                                         E.    EMI/RFI - Must be operational in close proximity                              to fuel pump                                                            F.    Fuel Tolerance - Sensor must be capable of meeting                            accuracy requirements for the following fuel types:                           TF1                                                                           TF2                                                                           UNLEADED GASOLINES                                                            100% INDOLENE HO-III                                                          PEROXIDE FUEL MIX                                                             METHANOL FUEL MIX                                                             CORROSIVE GASOHOL                                                             Additionally, the sensor must be capable of limited                           exposure to 2 RVP Fuel as well as not being adversely                         affected by exposure to legal and commercial fuels                            in the Asian, Mideast and European markets.                             G.    Underbody Contaminants - The sensor must withstand                            prolonged exposure to the following list of                                   potential underbody contaminants:                                       Engine Oil          Transmission Fluid                                        Power Steering Fluid                                                                              Coolant/Antifreeze                                        Brake Fluid         Windshield Wash Fluid                                     Transaxle/Differential Lube                                                                       Wheel Bearing Lube                                        Water               A/C Refrigerant                                           Snow, Ice           Acid Rain                                                 Car Wash Chemicals  Waxes, Paint Sealants                                     Steam Cleaning      Tire Cleaners                                             Engine Cleaning     Carpet Cleaners                                           Soft Drinks, Coffee, Etc.                                                     H.    Space/Size Requirements - The sensor shall be                                 contained preferably reasonably close to the following                        form factors:                                                           15 × 4 × 175 MM                                                                       15 × 4 × 400 MM                               10 × × 175 MM                                                                         10 × 6 × 400 MM                               I.    Electrical Requirements -                                                     Operational Voltage: 10.5 to 16.5 volts                                       Output voltage: 0 to 4.8 volts linearly related to                            measured level.                                                         J.    Mechanical Requirements - Sensor must survive a                               three feet vertical drop and still meet the electrical                        requirements.                                                           ______________________________________                                    

As mentioned above, to be suitable for use in automotive applicationsany liquid level sensor must meet the above requirements in addition tobeing capable of manufacture at a relatively low cost. This places anadditional constraint on the design of the liquid level sensing system.In summary then the fluid level sensing system must accurately measure avariety of materials (i.e., fluids) in a hostile environment as well asbeing capable of economical manufacture.

Accordingly, it is a principal object of the present invention toprovide a liquid level sensing apparatus incorporating a capacitiveprobe which senses the level of a liquid within a reservoir within whichthe capacitive probe is placed and which provides a sufficiently highlevel of accuracy which is not affected by changes in the dielectricconstant of the substance being monitored.

It is another object of the present invention to provide a liquid levelsensing apparatus which detects the level of a liquid within a fluidreservoir by detecting significant changes in the capacitance of acapacitive sensing probe having a plurality of segmented capacitorsformed longitudinally thereon along an axis of measurement of the probe.

It is still another object of the present invention to provide a liquidlevel sensing apparatus capable of differentiating the dielectricconstant of a substance at a plurality of points along a segmentedcapacitive probe disposed in the substance to thereby provide thecapability of determining not only the point at which the capacitiveprobe becomes disposed in air, but also changes in the dielectricconstant of the substance.

It is still another object of the present invention to provide a liquidlevel sensing apparatus which is economical to manufacture and suitablefor use in hostile environments such as those encountered in variousfluid reservoirs on a motor vehicle, and which meets or exceeds industryoperating requirements.

SUMMARY OF THE INVENTION

The above and other objects are provided by a digital liquid levelsensing apparatus in accordance with preferred embodiments of thepresent invention. The apparatus includes a segmented capacitive probehaving a plurality of independent input plates positioned longitudinallythereon along an axis of measurement of the probe and a common outputplate having a length sufficient to span the total length of the inputplates. The input plates are coupled to a plurality of independentoutputs of a controller. The controller generates a plurality ofsequential output signals to electrically excite each of the inputplates one plate at a time. As each input plate is electrically excitedit causes an output current to be coupled onto the common output plate.The magnitude of the output current depends on the capacitance, which inturn depends on the dielectric constant of the substance between theexcited input plate and the common output plate.

The common output plate is coupled to means for converting the currentoutput to a corresponding voltage. In the preferred embodiments thiscurrent converting means is comprised of a current to voltage amplifier.The current to voltage amplifier generates a voltage signal having amagnitude which "tracks" the output current coupled onto the commonoutput plate and generates a series of voltage signals representative ofthe output currents generated as each input plate is electricallyexcited by the controller. A peak voltage detector receives each of thevoltage signals and generates a peak voltage signal therefromrepresentative of the peak level of the output current coupled onto thecommon output plate from each one of the input plates.

Each of the peak voltage signals is applied to an input of thecontroller which compares the peak voltage signals generated on thecommon output plate from any given input plate with a predeterminedreference value stored in a first memory of the controller. Accordingly,the controller performs a plurality of comparisons, sequentially, of theoutput signals caused from the excitation of each input plate.

In the preferred embodiments each of the peak voltage output signals areconverted into a digital representation by an analog-to-digital (A/D)converter of the controller and stored in a second memory of thecontroller. In the preferred embodiments the predetermined referencevalue may be calculated from a reference input plate, such as an inputplate which is disposed in air even when the reservoir is full of fluid.Alternatively, this value may be a predetermined value which is storedin a read only memory of the controller, or alternatively in externalmemory such as an electrically erasable, programmable read only memoryalong with other historical information relating to each particularinput plate. One or more additional reference values relating tocompletely submerged input plates may also be stored in theabove-mentioned memories.

An appropriate program controls the comparisons between the outputsignals generated at the output plate from each of the input plates suchthat the output signal corresponding to each one of the input plates issubsequently compared against the reference value. The percentagecoverage of any partially submerged plate(s) is also rationalized by thecontroller from the one or more stored reference values. In this matterthe controller can detect even extremely small differences in the outputsignals resulting from excitation of any particular one of the inputplates. Accordingly, even input plates which are only partiallysubmerged in fluid cause an output signal which reflects this condition,and which can be readily detected by the controller. Thus, the fluid-airinterface can be readily and accurately determined. Variations in thedielectric of the fluid being sensed further do not adversely affect thedetection of the fluid-air interface.

In alternative preferred embodiments the apparatus includes anintegrator circuit for receiving an output signal from the controllerrepresentative of the total coverage of the capacitive plates of theprobe in fluid, and for providing an analog output signal indicative ofthe overall fluid level within the reservoir. In another alternativepreferred embodiment the apparatus includes a bi-directional serialinterface for enabling bi-directional communication between thecontroller and any external serial device. The controller of theapparatus may also optionally include means for generating a low leveloutput signal when the sensed fluid level falls below a predeterminedminimum level.

While the preferred embodiments of the present invention areparticularly well adapted for use with automotive vehicle applications,it will be appreciated that the invention could be used in connectionwith the sensing of virtually any liquid substance as well as a varietyof solid substances such as granular or plate type substances.Applications of the apparatus are further not limited to level sensingbut could just as easily include humidity sensing, position indicatingsystems and dielectric strength testing systems. The preferredembodiments of the apparatus provide a relatively low cost, low powerconsumption apparatus for effectively differentiating dielectricstrengths and for accomplishing accurate liquid level sensing.

In yet another alternative preferred embodiment, the apparatus of thepresent invention incorporates a temperature probe which sensestemperature changes in the fluid in which the capacitive plates of theprobe are disposed. Changes in the dielectric constant of the fluid dueto temperature changes can thereby be factored into the determination ofthe percentage of coverage of any particular input plate. This furtherincreases the accuracy of the liquid level determination.

BRIEF DESCRIPTION OF THE DRAWINGS

The various advantages of the present invention will become apparent toone skilled in the art by reading the following specification andsubjoined claims and by referencing the following drawings in which:

FIG. 1 is a block diagram of a digital liquid level sensing apparatus inaccordance with a preferred embodiment of the present invention, and,also showing several optional, yet desirable, circuit components forimplementing various optional functions;

FIG. 2 is an illustration of the capacitive probe showing an exemplaryform which the substrate may take and the overlapping of the inputplates;

FIG. 3 is a more detailed electrical schematic diagram of the apparatusof FIG. 1; and

FIG. 4 is flow chart of the series of steps performed by the controllerof the apparatus in detecting the presence and absence of fluid alongthe capacitive probe of the apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is shown a block diagram of a digital liquidlevel sensing apparatus 10 in accordance with a preferred embodiment ofthe present invention. The apparatus 10 generally includes a capacitiveelement array 14 which is disposed on a substrate 12. The array 14includes a plurality of input plates 14a₁ -14a₁₂ and a common outputplate 14b. The output plate 14b has a length which is sufficient to spanthe entire length of the adjacently positioned input plates 14a₁ -14a₁₂and both the common output plate 14b and plurality of input plates 14a₁-14a₁₂ are disposed longitudinally along an axis of measurement of afluid to be measured within a fluid reservoir.

Each of the input plates 14a₁ -14a₁₂ of the capacitive element array 14are coupled to independent outputs of a controller 16. In the preferredembodiments the controller comprises a microcontroller having aread-only memory (ROM) 18a and a random access memory (RAM) 18b thefunction of each of which will be described momentarily. It will beappreciated, however, that one or more external memory devices may beused in lieu of the memory devices 18a and 18b if for some reason thisis desirable to meet the needs of a particular application. In anotherpreferred embodiment an electrically erasable, programmable, read-onlymemory (EEPROM) 18c is used for storing reference values for each inputplate 14a₁ -14a₁₂. In connection with this embodiment an optionaltemperature sensor 19 may also be incorporated to provide indications ofchanges in temperature of the fluid in which the array 14 is disposed,which in turn can indicate changes in the dielectric constant of thefluid. By taking into account the changes in the dielectric constant dueto temperature, an even more accurate determination can be made as tothe percentage coverage of each input plate 14a₁ -14a₁₂.

In the preferred embodiments the controller 16 comprises a SGSmicrocontroller which includes serial communications ports RX 20 and TX22, and a plurality of output ports 24-46 which are electrically coupledto the input plates 14a₁ -14a₁₂. The controller 16 further includes afirst analog-to-digital input 48 in communication with an internalanalog-to-digital (A/D) converter, a pulse width modulated (PWM) output50, a second analog-to-digital input 52 and an output port 54.

The common output plate 14b of the capacitive element array 12 iscoupled to an input of a current to voltage amplifier 56. The amplifier56 has its output coupled to an input of a peak voltage detector circuit58. The output of the peak voltage detector circuit 58 is in turncoupled to the A/D input 48 of the controller 16.

The PWM output 50 of the controller 16 is coupled to an integratorcircuit 60. The output of the integrator 60 is in turn coupled to abuffer/driver circuit 62 which generates an analog output level signalto an external analog display device 64. The output of the integrator 60is shown being coupled back to the A/D input 52 of the controller 16such that a digital signal can be developed representative of the analoglevel output signal from the integrator 60. While circuit 60 has beenreferred to for convenience as an "integrator" circuit, it will beappreciated that this circuit in fact forms a low pass filter with acut-off frequency far below the repetition rate of the PWM output 50.This causes the circuit to function as an "averaging" circuit with anoutput voltage equal to the pulse amplitude times the ratio of the pulsewidth/repetition rate.

The output 54 of the controller 16 is coupled to an optional drivercircuit 66 for generating a low fluid level warning output signal. Thus,if the controller 16 determines that the detected fluid level is below apredetermined lower limit, the controller 16 generates a signal onoutput port 54 which driver circuit 66 uses to generate a warning to anoperator of a vehicle or other system with which the apparatus 10 isbeing used that a particular fluid level is below an acceptable lowerlimit. While this function of the controller 16 is optional, it isexpected at the present time that it will be desirable in manyapplications, and particularly in those involving automotive vehicles,where it is desirable to provide a warning to an operator of the vehicleimmediately if a particular fluid level drops below a predeterminedlower limit.

The signal from output port 54 is particularly useful in connection withsensing the level of oil in an oil reservoir of a vehicle. In automotiveapplications, it is important that the oil level within the oil pan ofthe vehicle not be allowed to fall below a minimum predetermined level.If such an event occurs, it is very important that the operator of thevehicle be notified immediately by some visual or audible means.

The controller 16 is bi-directionally coupled to a serialreceiver/transmitter circuit 68 via the RX and TX outputs 20 and 22,respectively. The serial receiver/transmitter circuit 68 essentiallycomprises a serial interface circuit which allows the controller 16 tocommunicate with another external controller, such as an engine controlmodule of an automotive vehicle, to thereby enable a signalrepresentative of the sensed fluid level to be communicated to theexternal device and other information to be communicated to thecontroller 16.

Referring to FIG. 2, the apparatus 10 is disposed on the substrate 12such that the capacitive element array 14 is disposed on an elongatedportion 12a of the substrate 12 within the fluid when positioned in areservoir. It is anticipated that in most instances all the input plates14a₁ -14a₁₂ will be disposed in fluid when the reservoir is completelyfull. In the preferred embodiments the input plates 14a₁ -14a₁₂ are eachformed in the shape of a parallelogram and positioned such that portionsof adjacent ones of the plates overlap slightly. This provides platesurfaces along the entire length of the input plates, thus eliminatingthe "gaps" that would otherwise exist between adjacent input plates 14a₁-14a₁₂.

Referring again to FIG. 1, a description of the operation of theapparatus 10 will now be provided. Initially, the ROM 18a of thecontroller 16 will include a predetermined "air plate" reference valuecorresponding to the output current coupled onto the common output plate14b when any input plate is disposed in air. A predetermined "fullplate" reference value will also be stored in the ROM 18a correspondingto an approximate expected output from the output plate 14b producedfrom exciting an input plate which is completely submerged in fluid.

The controller 16 sequentially applies a very short duration DC voltagepulse to each one of the input plates 14a₁ -14a₁₂, one at a time. Forexample, when the input DC pulse is applied to input plate 14a₂, anoutput current is coupled onto the common output plate 14b. The outputcurrent will vary in magnitude depending on the capacitance of thecapacitor formed between input element 14a₂ and common output plate 14b.Since the dielectric of air differs significantly from that of liquids,if the input plate 14a₂ happens to be disposed in air the output currentcoupled onto the common output plate 14b will be of a lower magnitudethan the current that would be coupled thereon if the input plate 14a₂had been submerged in liquid. Thus, the output current coupled onto thecommon output plate 14b as a result of the input signal applied to eachone of the input plates 14a₁ -14a₁₂ provides an indication as to whethera particular input plate 14a₁ -14a₁₂ is disposed in air, submerged influid, or at least partially disposed in fluid.

The controller 16 applies the DC input excitation pulses to the inputelements 14a₁ -14a₁₂ sequentially such that a series of independentoutput currents are coupled onto the common output plate 12 and input tothe current voltage amplifier 56. In some instances each of the inputplates 14a₁ -14a₁₂ may not need to be pulsed one at a time. For example,a fast response time algorithm may be used to pulse three of the inputplates virtually simultaneously once a partially submerged plate isdiscovered to thereby allow a plurality of outputs from a correspondingplurality of input plates to be "tracked."

Amplifier 56 generates a series of independent analog voltage signalswhich each represent the output current coupled onto the output plate14b by a particular one of the input plates 14a₁ -14a₁₂. It is importantto note that the common output plate 14b is not allowed to change itsvoltage potential while input pulses are independently applied to eachof the input plates 14a₁ -14a₁₂. A changing voltage on the common outputplate 14b could potentially cause measurement errors due to currentcoupling onto other capacitors and other stray parasitic capacitances.

The peak voltage detector circuit 58 generates a series of peak voltagesignals which represent the output signals coupled onto the commonoutput 14b as a result of excitation of each one of the input plates14a₁ -14a₁₂. The controller 16 converts these peak voltage signals intoa series of corresponding digital values and stores same in the RAM 18b.The controller 16 then, through the control of appropriate software,develops a table of differences from the output currents caused byexcitation of all of the input elements 14a₁ -14a₁₂. Put differently, atable of digital values is produced corresponding to the outputsgenerated by exciting the input plates 14a₁ -14a₁₂. The controller 16,through its software, then proceeds to compare the differences of thedigital output values produced by the input plates 14a₁ -14a₁₂ with thepreviously stored "air plate" and "full plate" reference values. Thesoftware program looks for a predetermined, significant difference inthe magnitude of the output signal caused by each input plate whencompared to the air plate reference value. When the controller detects apredetermined significant difference between a digital value and the airplate reference value, this is an indication that the present digitalvalue corresponds to an output signal generated by a specific, knowninput plate which is at least partially covered with fluid. As thecontroller 16 continues to make successive comparisons and determinesadditional differences between each subsequent digital value and the airplate reference value stored in the ROM 18a, it readily determines whichof the input plates are completely disposed in fluid based on the degreeof variation of each digital value from the air plate reference value.The percentage of coverage (i.e., submergence) of any particular inputplate can then be approximated based on its comparison to the air platereference value and the full plate reference value. Since the relativeposition of the apparatus 10 within the reservoir is predetermined, inaddition to the shape of the reservoir and the overall volume of thereservoir, detecting the precise point along the axis of measurement atwhich the liquid-air interface is present allows the overall level ofthe liquid within the reservoir to be readily extrapolated.

It is anticipated future versions of the apparatus 10 will incorporatesoftware enhancements to compensate for irregular geometries of thecontainment vessel or reservoir. A look-up table which assigns scalingvalues to segments according to their position along the array 14 willprovide the required correction factors to compensate for odd-shapedreservoirs.

In an alternative preferred form of operation, the predetermined airplate and full plate reference values are used together with real timecalculating of "average" air and full plate reference values. In thisform of operation the software repeatedly calculates an average airplate value by taking the outputs produced from input plates 14a₁ -14a₁₂which are determined (by comparison with the air plate reference value)to be disposed entirely in air and obtaining an average air platereference value from these input plates 14a₁ -14a₁₂. Similarly, anaverage full plate reference value is obtained from those input plates14a₁ -14a₁₂ which are determined (initially by comparison to thepredetermined air plate or predetermined fullplate reference value) tobe completely submerged in fluid. This is done by obtaining the averageoutput produced by the input plates 14a₁ -14a₁₂ which are completelysubmerged in fluid. These average air plate and full plate referencevalues are then used to rationalize, more accurately, the percentagecoverage of any partially submerged input plate 14a₁ - 14a₁₂. Thisaveraging may be expressed by the following formula: ##EQU1## where Vmeasured=measured voltage output value where V ref_(low) =referenceoutput voltage for a free air input plate; and

where V ref_(high) =reference output voltage for a fully submerged inputplate

These average reference values are repeatedly calculated, in real time,to take into account small changes in the dielectric constant of thefluid. The percentage coverage value may then be multiplied by asuitable scaling or weighting factor relating to that particular inputplate's relative position in the reservoir and/or the volume of fluid inthe reservoir.

In yet another preferred form of operation the EEPROM 18c is used tostore actual full plate and air plate reference values for each singleinput plate 14a₁ -14a₁₂. These actual reference values are repeatedlyupdated during operation such that when a particular input plate 14a₁-14a₁₂ is determined to be at least partially submerged, its percentageof submergence can be even more accurately determined. If thetemperature sensor 19 is used with this embodiment, then the controller16 can further modify the actual air plate and full plate referencevalues in accordance with changes in the temperature of the fluid, whichtemperature changes can affect the dielectric constant of the fluid. Tothis end a suitable look-up table stored in some memory of the apparatus10 could be accessed by the controller 16 to obtain appropriatecorrection factors to be applied based on specific changes in the sensedtemperature.

Referring to FIG. 3, a more detailed schematic diagram of the apparatus10 is shown. In the drawing of FIG. 3 a second capacitive output plate14c is included on the substrate 14. This output plate 14c is maintainedat ground and, in effect, a shield to protect the common output plate14b from noise currents by shunting any such currents to ground, forsystems using a common ground. Preferably, the printed circuit boardupon which the apparatus 10 is disposed has an internal ground plane tofurther eliminate noise transfer.

With further reference to FIG. 3, the current to voltage amplifier 56comprises an operational amplifier 56a incorporating a feedback resistorR1 coupled to its inverting input to thereby provide a negative feedbackloop. Its value is determined based on initial open plate (i.e., aninput plate in air) capacitance. Resistors R6 and R3 form a voltagedivider for generating a suitable input voltage to the non-invertinginput of the operational amplifier 56a. It should be noted that adesirable characteristic of the amplifier 56 is that its output voltageis only affected by input elements which source or sink current from theinverting node (input) of the operational amplifier 56a, which is atvirtual ground potential. Since the output plate 14b is tied to thisinput, many problems otherwise encountered with a common elementapproach are eliminated. In operation, the adverse loading effectscaused by the parasitic capacitance of the un-driven input plates 14a₁-14a₁₂ is ignored because of this principle.

The peak voltage detector circuit 58 is formed by diode D2 coupled inparallel with capacitor C5. The peak detector 58 functions to capturethe peak amplitude of the output pulse generated on the output plate14b, for each input pulse generated by the controller 16, long enoughfor the controller 16 to read the signal with its internal A/Dconverter. Diode D3 prevents gain from positive currents into theamplifier 56 which would try to otherwise make the output of theamplifier 56 go negative. The integrator 60 of FIG. 1 is shown in FIG. 3as being represented by resistor R5 and capacitor C6. Buffer/drivercircuit 62 is represented by operational amplifier U3B and resistor R4.The driver circuit 66 is represented by resistor R6 and NPN transistorQ1. The receive (RX) port 20 and the transmit (TX) port 22 of thecontroller 16 are shown as ports 27 and 26, respectively, of thecontroller 16. A ceramic resonator 72 is coupled to the clock inputs ofa controller 16 to provide a timing signal of preferably about 2-8 Mhzto the controller 16.

Referring now to FIG. 4, a flowchart is provided which furtherillustrates the steps of operation of the preferred form of operation ofthe apparatus 10. At step 74 the controller 16 is initialized. At step76 the controller cycles through all, or at least a selectedsub-plurality, of the input plates 14a₁ 14a₁₂ to obtain a digital outputvalue associated with each input plate 14a₁ -14a₁₂. At step 78 thecontroller 16 mathematically derives the average air plate (i.e., lowplate) reference value from all the input plates which are disposedcompletely in air. As indicated at step 80, the controller thenmathematically derives the average full plate reference value from allthe input plates 14a₁ -14a₁₂ which are completely submerged in fluid.

At step 82, the controller determines any input plate 14a₁ 14a₁₂ whichis not entirely disposed in air or which is not completely submerged influid and further determines the percentage coverage of the input plate(i.e., the percentage of the plate which is submerged in fluid). Thecontroller then reads the temperature sensor 19, if this component isbeing used, as indicated at step 84 and then prepares to output a signalto at least the PWM controller output 50, as indicated at step 86.

Optionally, the controller 16 may calculate the full/empty differentialvalue between a given input plate's outputs when the input plate iscompletely submerged in fluid and completely disposed in air, asindicated at step 88. This provides an indication of the dielectricstrength of the fluid itself.

Next, the software prepares the data obtained for serial output fromport 22, as indicated at step 90. The EEPROM 18c is then updated if thiscomponent is being used, as indicated at step 92.

By differentiating the dielectric strength at segmented capacitive inputelements, the apparatus 10 of the present invention lends itself well toa variety of applications wherever the dielectric strength of capacitiveelements of a probe varies with the level of the substance beingdetected. Applications of the methodology of the present inventioninclude fluid, gaseous and solid dielectric strength measurements.Specific applications for which the apparatus 10 is particularly welladapted include liquid and solid (granular or plate type substances)level sensing, humidity sensing, and use as a dielectric strength testerand position indicator.

Those skilled in the art can now appreciate from the foregoingdescription that the broad teachings of the present invention can beimplemented in a variety of forms. Therefore, while this invention hasbeen described in connection with particular examples thereof, the truescope of the invention should not be so limited since othermodifications will become apparent to the skilled practitioner upon astudy of the drawings, specification and following claims.

What is claimed:
 1. An apparatus for detecting a fluid level within afluid reservoir within which a capacitive element array is disposed,wherein the capacitive element array includes a plurality of independentinput plates spaced longitudinally along an axis of measurement and asingle output plate disposed generally parallel to said input plates andhaving a length sufficient to span all of said input plates, saidapparatus comprising:controller means for sequentially applying an inputsignal to each one of said input plates sequentially, one at a time, tothereby electrically excite each said input plate; current to voltageamplifier means responsive to said output plate for generating a voltagesignal representative of the current coupled onto said output plate eachtime one of said input plates is electrically excited by said controllermeans; peak detector means responsive to said current to voltageamplifier means for generating a peak voltage signal representative ofthe peak magnitude of the voltage output signal from said current tovoltage amplifier means each time one of said input plates iselectrically excited by said controller means; means for storing eachone of said peak voltage signals associated with the current coupledonto to said output plate for each input plate excited by saidcontroller means; said controller means including means for comparingsaid peak voltage output signals generated from the excitation of saidinput plates, one of said input plates at a time in sequential fashionagainst a reference value, and for determining the difference in each ofsaid peak voltage output signals from said reference value, to therebydetect a change in the dielectric from any one of said input platesrelative to said reference value, to thereby detect a liquid/airinterface within said fluid reservoir from which a level of said liquidcan be determined.
 2. The apparatus of claim 1, further comprisingintegrator means for integrating an output signal generated by saidcontroller means representative of said peak voltage output signals forproviding an analog indication of a level of said liquid within saidreservoir.
 3. The apparatus of claim 1, further including interfacemeans in communication with said controller means for coupling an outputrepresentative of said liquid level to an external device.
 4. Theapparatus of claim 1, wherein said controller means includes means forgenerating an output signal when said liquid level within said reservoiris determined to be below a predetermined lower level limit.
 5. Theapparatus of claim 1, further comprising an electrically erasable,programmable, read-only memory for storing information for each one ofsaid input plates relating to outputs generated when each said inputplate is completely disposed in air and completely submerged in fluid.6. The apparatus of claim 1, further comprising means for sensing atemperature of said fluid and providing a signal representative of saidsensed temperature to said controller.
 7. A liquid level sensingapparatus for detecting a fluid-air interface level and a fluidreservoir containing a fluid therein;a capacitive probe positionedwithin said reservoir longitudinally along an axis of measurement ofsaid fluid, said probe comprising a plurality of independent inputplates positioned in a common plane longitudinally along said probe andalong said axis of measurement and a common output plate having a lengthsufficient to span a total accumulated length of said input plates; acontroller for generating a plurality of input pulses to each of saidinput plates sequentially, only one said input plate at a time, tothereby couple a sequential series of independent current signals ontosaid common output plate; a current to voltage amplifier coupled to saidcommon output plate for generating a plurality of independent voltagesignals corresponding to the magnitude of each one of said currentsignals coupled onto said common output plate; a peak voltage detectorcircuit responsive to said current to voltage amplifier for generating aseries of peak voltage signals, each one of said series of peak voltagesignals corresponding to a particular one of said current signalscoupled onto said common output plate from a particular one of saidinput plates; said controller including means for comparing the peakvoltage signals from each of said input plates with a predeterminedreference value, sequentially, to detect for a predetermined differencein magnitude between each of said peak voltage signals and saidreference value; said predetermined difference in magnitude beingindicative of a change in a dielectric at a specific one of said inputplates from which change a liquid-air interface can be determined. 8.The apparatus of claim 7, wherein said controller includes means forgenerating pulse width modulated output signals from said peak voltageoutput signals which is representative of the percentage of said commonoutput plate and said input plates which are submerged in said fluid;andwherein said apparatus further includes an integrator responsive tosaid pulse width modulated output signals for generating an analogoutput level signal representative of said percentage of coverage ofsaid common output plate and said input plates by said fluid.
 9. Theapparatus of claim 7, further comprising a serial receiver/transmitterinterface circuit in communication with said controller; andwherein saidcontroller includes means for generating a serial output signalindicative of a percentage of area of said common output plate and saidinput plates which is submerged in said fluid at a given time, and fortransmitting said serial output to an external apparatus.
 10. Theapparatus of claim 7, wherein said controller includes means forgenerating a low level output signal when said controller determinesthat said liquid-air interface is below a predetermined minimum fluidlevel within said reservoir.
 11. The apparatus of claim 7, wherein saidinput plates are positioned along said axis of measurement such that aportion of each one of said input plates overlaps a portion of itsimmediately adjacent input plates.